1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging lens having a four-lens structure that is suitable for forming an image of an object on an image-taking surface of an image sensor element, such as a charge-coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), mounted on a portable computer, a television phone, a portable phone, a digital camera, a monitoring camera for a vehicle, and the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for cameras that utilize an image sensor element (solid image sensor element), such as the CCD and the CMOS, mounted on a portable computer, a television phone, a portable phone, a digital camera, a monitoring camera for a vehicle, and the like. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
A solid image sensor element having a high resolution of about 300 thousand pixels, called video graphics array (VGA), and a solid image sensor element having a higher resolution of one million pixels or more are being used as the solid image sensor element mounted on cameras such as those described above.
Therefore, like the cameras, the imaging lens used in such cameras is required to be small and light and to have a high resolution. Conventionally, a four-lens structure lens system using four lenses, such as those described in Patent Literature 1 to Patent Literature 3, is used to meet these demands.    [Patent Literature 1] Japanese Patent Unexamined Publication    [Patent Literature 2] Japanese Patent Unexamined Publication 2004-341512    [Patent Literature 3] Japanese Patent Unexamined Publication 2002-365531
It is important that the monitoring camera for a vehicle and the like, in particular, can also image a subject in a dark environment, such as outside at night, with adequate exposure. Therefore, the camera is required to have a high dynamic range (dB). A lens mounted on such a camera having a high dynamic range is preferably a bright lens with a large diameter in which reduction in the amount of light is minimal even at its periphery.
However, in the lens system described in Patent Literature 1, a diaphragm is disposed closest to an object side. Therefore, although the lens system is ideal for actualizing a compact lens system in which an incident angle of a light beam incident on an image-taking surface is small, asymmetry of the lens system increases. Therefore, from the perspective of aberration correction, the configuration is not particularly suitable for a large diameter lens system having a large effective diameter. This is because many kinds of aberrations (such as coma aberration and astigmatism) are affected by the effective diameter of a lens.
In the lens system described in Patent Literature 2, a first lens is a lens having a strong convex surface that faces the object side. Therefore, peripheral vignetting (aperture efficiency) may decrease.
Here, when an effect caused by distortion is ignored, the amount of light in the periphery (referred to, hereinafter, as peripheral light amount) is calculated by the fourth power of the cosine of the angle of field (cos4ω) being multiplied with a vignetting value. Therefore, in the lens system described in Patent Literature 2 having a small peripheral vignetting value, decrease in the peripheral light amount is difficult to suppress.
Moreover, in the lens system described in Patent Literature 3, a second lens is a biconcave lens. Therefore, although the lens system is effective in terms of increasing negative power and reducing a Petzval sum, the incident angle of the light beam entering the image surface side away from the diaphragm increases, and various aberrations such as astigmatism frequently occur. Therefore, the lens system is particularly disadvantageous for a large diameter lens system having a large effective diameter.
In this way, conventionally, in a bright, large-diameter lens system, it is difficult to maintain balance among suppressing the decrease in the peripheral light amount, reducing the size and weight of the lens system, and maintaining excellent optical performance by the incident angle of the light beam incident on the image-taking surface being relaxed or the various aberrations being corrected.